This invention relates to an optical transmitter, and more particularly to an optical transmitter with a laser diode of low threshold current density.
Recently, optical transmission technology has been developed remarkably and ultra high speed optical transmission technologies by way of a single mode optical fiber, an optical transmission device for a long wave laser light, are investigated for realizing a large capacity long distance transmission.
Especially for providing a wide area communication network used for delivering multimedia services such as video picture, audio sound or information data, a high-speed, stable and practical optical transmitter is expected.
In the wide area communication network, not only for the trunk lines, where time division multiplex signals of several giga-bits per second are transmitted requiring a high-speed and wide band transmitter, but also for subscriber lines, these optical transmission technologies are attracting considerable attention as a feasible strategy for providing a high quality subscriber line interface for multimedia services.
Basic functions needed of the optical transmitter/receiver are so called 3R, that is, resharping (or equivalent amplification), retiming and regeneration. In the regeneration among them, the optical transmitter plays an important role to send out final transmission signals.
Therefore, cost reduction of the optical transmitter is a hot theme for developement of subscriber communication systems.
As for the cost reduction of optical transmitters, a regulation free setting of bias current of a laser diode, a simplification of a driving circuit of the laser diode or a reduction of power consumption of the circuit has been investigated, and recentry as a result of developement of a low threshold laser diode, a zero-bias modulation is realized, which is reported in "Zero-bias Modulation of extremely Low Threshold 1.3 .mu.m DFB-PPIH laser diode", Ohkura et al, Optical Communication Society 88-15, pp. 37 to 41.
FIG. 5 illustrates a circuit configuration of a conventional optical transmitter using a comparatively low threshold current density laser diode as its optical source. Input signals are amplified by a laser diode driver (hereafter abbreviated to a LD driver) 51 to a signal level sufficient for activating a laser diode (hereafter abbreviated to a LD) 52, which sends out optical outputs 54 according to driving pulses 53 supplied from the LD driver 51. Wave forms of a driving pulse 53 and a corresponding optical output 54 are shown in FIG. 6.
In general, when a LD is used without bias, it causes a radiation delay, which depends on threshold value of the LD and becomes longer with the higher threshold value. Therefore, a radiation delay longer than a signal period may cause information defects. This is an important problem of the LD.
The radiation delay td of a LD, used with bias lower than its threshold value, is given by a following equation; EQU td=Ts.times.ln{Jp/(Jp-Jth+Jb)} (1)
where, Ts, Jth, Jb and Jp denote carrier lifetime, threshold current value, bias current value and driving current value respectively, the driving current value defined as a current value difference between a peak current and the bias current.
From equation (1), the radiation delay td of about 250 ps is given in a LD with a threshold current value Jth of 3.5 mA activated by a driving current Jp of 40 mA without bias current. So, in a conventional optical transmitter as shown in FIG. 5 with a comparatively low threshold LD 52, the optical output 54 is delayed from the driving pulse 53 as shown in FIG. 6.
The radiation delay td of a LD represented by equation (1) is on condition that the LD is activated with a driving pulse after a certain interval. The radiation delay td becomes shorter when the LD is activated with a driving pulse following other preceding pulses than that after the interval, since carrier density in active layers of the LD 52 widely fluctuates depending on pulse patterns of the driving pulse 53.
Consequently, a radiation delay td1 for a first pulse #1 of the optical output 54 in FIG. 6, which equals td obtained from the equation (1), is longer than the next radiation delay td2 for a following pulse #2.
This fluctuation of the radiation delay results in jitters of rising edges of the optical output 54, and when transmission speed becomes high, more than 1 Gb/s for example, influences of the radiation delay td and the jitters become important compared to pulse widths, degrading the transmission characteristic and limiting the errorless transmission speed.
In prior art reducing the radiation delay td, a LD bias circuit 74 is provided as illustrated in FIG. 7 for supplying a bias current to the LD 52 as shown in FIG. 8. However, the bias current of the prior art cancels the merit of low dissipation of the comparatively low threshold LD 52 and complicates the circuit configration of the optical transmitter.